The invention relates to a process for preparing a carboxylic acid.
The beta-hydroxy acid, 3-hydroxypropionic acid (3HP) can be dehydrated to its corresponding unsaturated acid, acrylic acid (AA), under acidic conditions at elevated temperatures, generating water as a by-product. In the preparation of 3HP by fermentation of sugar(s), the necessity of operating the fermentation at or near neutral pH leads to formation of a 3HP salt, most commonly the ammonium salt (A3HP) as an aqueous solution. Gaseous or aqueous ammonia is used as a base titrant throughout the fermentation process to form A3HP, and the A3HP is converted to its free-acid form, namely 3HP, before it is converted to acrylic acid in a dehydration reaction. Conversion of A3HP to 3HP can be accomplished by various methods including: (A) mineral acid acidification, e.g., with sulfuric acid (H2SO4), or phosphoric acid (H3PO4); (B) same as (A) followed by stripping or evaporation of water and some volatile contaminants and their subsequent collection as a condensate (the 3HP remains in the liquid phase of the process); (C) solvent-based or solventless thermal salt splitting; (D) alkyl-amine assisted solvent extraction followed by back-extraction of the free-acid into water; and (E) the use of an electrolytic membrane device.
Subsequently, the 3HP free-acid produced in any of the above schemes may be dehydrated to acrylic acid by a strong mineral acid catalyst, such as concentrated sulfuric acid. Both heterogeneous solid-acid catalysts and homogeneous mineral acid catalysts have been described in the patent and open literature. In all of these schemes, the presence of residual ammonium ion ((NH4)+) is possible. Scheme (A) represents the extreme case where all of the ammonium ion remains and is present as ammonium sulfate ((NH4)SO4) or ammonium bisulfate ((NH4)HSO4). Lesser amounts of ammonium ion are expected to accompany the free-acid in the other schemes. When the free-acid-containing effluent of any of the above schemes is fed to concentrated sulfuric acid, 3HP is dehydrated to acrylic acid that can be removed by distillation, while any residual ammonium ion is converted to ammonium bisulfate in the liquid phase. The steady state sulfate salt composition of the dehydration reactor effluent is established by the mole ratio of sulfuric acid to ammonium ion. The salt composition can vary from predominantly ammonium sulfate (AS) at lower mole ratios (e.g., less than (<) 0.5), AS and ammonium bisulfate (ABS) mixtures at intermediate mole ratios (e.g. 0.7), and mixtures of ABS with excess sulfuric acid at mole ratios greater than (>) 1.0.
When an excess of sulfuric acid is used as the dehydration catalyst for the dehydration of hydroxycarboxylic acids to produce carboxylic acids, disposal of the reactor bottoms is difficult due to the corrosive nature of the stream. In one commercially practiced process, an acid recovery combustion process is used to convert sulfates into sulfuric acid and recycle the acid back to the dehydration reactor. The ammonia in the bottoms stream of the process, however, is not recovered. A sulfuric acid recovery unit employs a hazardous process, requires significant capital investment, and has significant environmental, health, and safety concerns.
Alternatively, the ammonia can be recovered at the cost of losing the sulfuric acid. For example, the gypsum process, in which gypsum (CaSO4) is produced from the reaction of calcium oxide (CaO, lime) with ammonium sulfate, provides such an alternative. The recovered ammonia is then fed back to the fermenters. The gypsum, however, has too high of an organics content to be sold, and there are limited landfill options because as gypsum breaks down it generates hydrogen sulfide (H2S), which is toxic and causes corrosion of the turbines used to generate energy from landfill gases.
In view of the disadvantages of the existing processes, an improved process for the dehydration of hydroxycarboxylic acids and their ammonium salts to carboxylic acids is desirable. In addition, an alternative to the conventional sulfuric acid catalyst system from which both ammonia and the dehydration catalyst can be recovered and that provides a simpler and milder catalyst regeneration process is also desireable.